This invention is directed to voltage controlled oscillators and a method for tuning such oscillators.
In the field of airborne radar systems, it is often desirable to employ pulse doppler radar which requires a high dynamic range. These types of radar systems are used in an airplane for the on-board detection of a target. Such systems require the use of a clutter oscillator to improve detection of a target from among a large number of false signals or "clutter" produced by the ground. New system mechanizations for ground moving target modes require very accurate clutter positioning to less than 5 Hz accuracy, while maintaining the capability for look-to-look offset switching. This required resolution is much higher than the 50 Hz resolution provided by currently available systems. Compounding this dramatically improved performance requirement is a reduction in the amount of system time available for calibration of the clutter positioning function, partly due to faster scan times for phased array antenna scanning.
FIG. 1 is a block diagram of a typical prior art radar system which employs a calibrated/curve fitted voltage-controlled oscillator technique. The radar system of FIG. 1 includes a stable local oscillator (STALO) 20 for providing a transmit drive signal to a transmitter 22 and local oscillator signals LO.sub.1 and LO.sub.2 to a receiver 24. Based on the transmit drive signal, the transmitter 22 provides an output signal to a circulator 26 and then to an antenna 28. The receiver 24 receives incoming signals through the antenna 28 and the circulator 26. A digital signal processor 30 processes the received signals and provides an output signal to a general purpose radar computer 32 which is connected to a display 34. The digital signal processor 30 measures the frequency of the signal provided by the receiver 24 and determines whether a target or the ground has been detected.
Due to the requirements of the prior art radar system, it is necessary to calibrate an oscillator which is included in the STALO 20 by using the receiver 24 and the signal processor 30. The receiver 24 receives the local oscillator signals LO.sub.1 and LO.sub.2 from the STALO 20 and produces a tuning voltage V.sub.T to tune the oscillator in the STALO 20. The time required by the receiver 24 and the signal processor 30 to calculate and send the tuning voltage V.sub.T reduces the time available to the radar system for its primary function of finding targets. As a result, the resolution of currently available systems has been limited to 50 Hz.
If the prior art system described in FIG. 1 were to be modified to improve resolution to 5 Hz, this would require ever increasing calibration time and repetition rates to achieve such improved performance. In order to produce 5 Hz resolution using this type of system, it would require that one second of time be used as often as once every ten seconds to calibrate the oscillator in the STALO 20 as a result of variations in the oscillator output due to wide variations in temperature conditions. Since the use of such a large amount of time for calibration purposes takes away from the ability of the radar system to perform its primary function of detecting targets, there is a need in the art for an oscillator circuit having improved frequency resolution without requiring a large amount of calibration time.
The inventors of the subject application have considered the use of indirect frequency synthesis in order to provide frequency accuracy. Indirect frequency synthesis is an efficient means for generating many channels. However, frequency synthesis requires a phase-locked loop having a response time on the order of the reciprocal of the channel spacing. This precludes the use of a phase-locked loop in a fast switching application of the type required in advanced radar systems, where 5 Hz frequency channels must be locked up within at least approximately 500 microseconds. Specifically, the frequency synthesis approach takes approximately 30 milliseconds to lock up, whereas advanced radar systems preferably require lock up in 10 to 100 microseconds. Indirect frequency synthesis is also limited by sideband modulations at the channel spacing, particularly when used in STALOs which require 90 to 100 dB spurious. Thus, while the indirect frequency synthesis approach provides an improved oscillator, it is not practical for advanced radar systems because of the narrow loop bandwidth required in such systems.
In summary, there is a need in the art for an oscillator circuit which is capable of use in advanced high resolution radar systems. In particular, there is a need for an oscillator circuit capable of 5 Hz resolution within a relatively short lock up time, and for a method of tuning such an oscillator circuit.